The present invention relates to a control device for a vehicle mounted generator and, particularly, to such device by which an undesired energization of an alarm lamp due to a disconnection of an electrical load of the vehicle from the generator is prevented so long as the generator system is operating normally.
FIG. 1 is a circuit diagram of a prior art control device for a vehicle mounted generator, in which a generator 1 which is driven by an engine (not shown) of the vehicle includes an armature winding 101 and a field winding 102. A fullwave rectifier 2 having output terminals 201, 202 and 203 is connected to the generator 1 to rectify the a.c. output voltage thereof. The main output of the generator 1 is derived from terminal 201 amd output terminal 202 is connected to field winding 102, a voltage regulator 3 and an alarm circuit 4 to supply an output voltage to them. Terminal 203 is grounded.
The voltage regulator 3 serves to regulate the output voltage of the generator 1 to a predetermined value and includes a voltage divider composed of a series connection of resistors 301 and 302 connected between the output terminal 202 and ground. The junction between the series connected resistors 301 and 302 is connected through a Zener diode 303 to the base of an input transistor 304 of the voltage regulator 3. The Zener diode 303 serves to detect the output voltage of the generator 1.
The emitter of the input transistor 304 is grounded and the collector thereof is connected to the base of an output transistor 305 to the voltage regulator 3. The base of transistor 305 is connected through a base resistor 306 to output terminal 202. The emitter and collector of transistor 305 are grounded and connected through a diode 307 and the field winding 102 to output terminal 202, respectively. That is, diode 307 which serves to absorb surges of the field winding 102 is connected in parallel to the field winding 102. Transistor 304 is on-off controlled by the Zener diode 303 and transistor 305 is on-off controlled by transistor 304 to on-off control the current of the field winding 102.
The alarm circuit 4 is provided to monitor the output voltage of the generator 1 and includes a voltage divider composed of series connected resistors 409 and 410 connected between output terminal 202 and ground. The junction between the resistors 409 and 410 is connected to an inverted input of a comparator 406.
A non-inverted input of the comparator 406 is connected to the junction between series connected resistors 407 and 408 constituting a voltage divider connected between a constant voltage source A, which is established when a key switch 7 is closed to start the engine, and ground, to provide a reference voltage to the non-inverted input.
The output of the comparator 406 is connected through a Zener diode 404 to a base of a transistor 405 and through a resistor 403 to the output terminal 202. The comparator 406 is enabled upon the closure of the key switch 7.
Transistor 405 is turned on when the Zener diode 404 is turned on, and has a grounded emitter and a collector connected to the base of an output transistor 401 and through a resistor 402 to a point P.sub.1 between the key switch 7 and a reverse current blocking diode 8. The output transistor 401 has a grounded emitter and a collector connected through an alarm lamp 10 to the point P.sub.1 which is connected through diode 8 and a resistor 9 for the initial energization of the field winding 102 to the output terminal 202.
Point P.sub.1 is grounded through the key switch 7 and a battery 5 which is mounted on the vehicle. Electrical loads which are depicted as a single load 5 are connected in parallel to the battery, whose positive terminal is connected to the output terminal 201.
In operation, when the key switch 7 is closed to start the engine, an initial energizing current flows from the batttery 5 through the key switch 7, the reverse current blocking diode 8 and the resistor 9 to the field winding 102, and thus the generator 1 becomes ready to generate.
The potential at the output terminal 202 of the rectifier 2 under such condition is a fraction of the battery voltage determined by the internal resistance of the field winding 102, and is relatively low.
On the other hand, the alarm circuit 4 is supplied at the base of the transistor 401 with the battery voltage through the key switch 7 and the resistor 402 to turn the latter transistor on to thereby light the alarm lamp 10.
When the engine starts and the generator 1 starts to generate an output voltage, the latter is applied through the resistor 403 to a cathode of the Zener diode 401. When the output voltage increases, the Zener diode 404 is turned on to turn transistor 405 on and transistor 401 off, to thereby extinguish the alarm lamp 10, indicating a commencement of an ordinary power generation of the generator 1.
On the other hand, the voltage regulator 3 detects the output voltage by the resistors 301 and 302 and the Zener diode 303 and, when the output voltage exceeds a value preset by these elements, the Zener diode 303 is turned on and transistor 304 is also turned on. When the output voltage is reduced below the preset value, the Zener diode 303 and hence transistor 304 are turned off.
Transistor 305 is on-off controlled by transistor 304 to on-off control the field current of the winding 102, to thereby regulate the output voltage to the preset value in a well known manner.
If the voltage regulator 3 is out of operation for some reason, the output voltage of the generator may increase abnormally. Under such an abnormal condition, the abnormal output voltage is detected by resistors 409 and 410 and the comparator 406 of the alarm circuit 4 and, when the detected voltage is higher than the reference voltage of the comparator 406, the output of the latter drops to an "L" level upon which Zener diode 404 and transistor 405 are turned off and transistor 401 is turned on to light the lamp 10.
That is, the alarm circuit 4 functions to indicate the out-of-control condition of the voltage regulator 3, i.e., an overvoltage at the output of the generator 1.
The reference voltage of the alarm circuit 4 is usually set higher by about 2 volts than the preset voltage of the voltage regulator 3.
On the other hand, when the electrical load 6 which is relatively large is connected to the battery when the generator system is operating normally, the output current of the generator 1 increases correspondingly to the load and a large field current is thus necessary to provide such a large output current flowing through terminal 201. When the load 6 is disconnected from the battery under such condition, the output voltage of the generator 1 is increased. When the increased output voltage exceeds the preset value of the voltage regulator 3, transistor 305 is turned off and thus the field current is cut off. However, since the field winding 102 is inductive, a large amount of inductive energy is produced. Further, since the field winding 102, together with the diode 307, forms a closed circuit, such inductive energy is consumed by the internal resistance of the field winding 102. The inductive current has a peak value before the cut-off time of the field current, and is attenuated at a rate determined by a LR time constant of the resistance and the inductance of the field winding 102.
That is, since current continues to flow through the field winding 102 for a short time after the current supply controlled by the voltage regulator 3 is cut off, the output voltage of the generator is further increased in this short time.
The higher the increasing rate of the output voltage the larger the field current, i.e., the larger the output current, before the field winding 102 is cut off by the voltage regulator, and the higher the rotational speed of the generator 1.
Thus, when a load 6 which is relatively large is disconnected from the battery, the output voltage of the generator 1 increases above the preset value of the voltage regulator 3 and, when the output voltage reaches the overvoltage detection level of the alarm circuit 4, the lamp 10 is lit momentarily which may give the operator of the vehicle a sense of uneasiness.